Studies in our laboratory have shown that the primate parvocellular red nucleus (pRN) receives a massive projection from all of the major motor and premotor zones of the fronto-parietal cortex. Since the outflow of the pRN is directed by way of the principal inferior olive toward the lateral cerebellum, which projects back toward the cerebral cortex, it is clear that the pRN and its corticofugal input are part of a large, cerebro-cerebellar communication system. It is not clear at present, however, precisely what functions are performed by this massive neuronal network. Previous lesion studies of the RN have been inconclusive, because fibers of passage were also destroyed. Similarly, our own preliminary studies have failed to reveal any correlation between the discharge of cortico-pRN or of pRN neurons and various aspects of well-learned, spontaneous movements by the alert monkey. It is possible, therefore, that this system is involved only in some 'higher-order' way in the programming of complex movements, or in those motor learning processes which allow for adaptation to unfamiliar or unpredictable motor requirements. We propose to test this and other possibilities, and to gather new information about this large communication system, by way of three basic sets of experiments. First, we will use a wide battery of motor tasks to obtain quantitative information about the deficits in motor programming, learning, and/or performance that are seen in the alert monkey after destruction of the pRN. In these experiments, a chemical lesioning method will be used that spares cerebello-thalamic fibers of passage. Second, we will use semi-chronic recording and stimulation techniques to examine the patterns of convergence onto single pRN neurons of fibers from various motor and premotor regions of the cerebral cortex. Finally, we will again use a battery of motor tasks and chronic, single neuron recording methods, to examine in the alert monkey the motor-related behavior of cortico-pRN and of pRN neurons. These experiments will be the first aimed directly at determining the fuctions of this major subdivision of the red nucleus and its massive cortical input, both of which are part of a cerebro-cerebellar network that reaches its highest level of development in man.